1. Field of the Invention
The present invention is related to a switch-mode self-coupling auxiliary power device, which is an assistant power device providing an enough voltage power to a controller under a low load.
2. Description of Related Art
According to the tendency of information appliances, in addition to powerful functions and low price, power saving is an important subject. For conforming to the requirement for power saving, many standards have are established; for example, GEEA of Europe stipulates that a power converter for a LCD TV should have a standby power of less than 1 W, and ENERGY STAR of U.S.A. also stipulates that a power converter with an output power ranging between 10 to 250 W should have a no-load power less than 0.75 W. Therefore, when improving the material used by the manufacturer for achieving a power-saving effect, the power converter is also an important key point in power saving. However, when the conventional power supplier containing a PFC (Power Factor Correction) is used with information appliances, conformation to the power saving standards does not always occur, so as to cause a lot of bother.
References are made to FIGS. 1A, 1B and 1C which are schematic views showing the circuits of the conventional power converter. The power converter circuit utilizes an isolation function of a transformer T to divide the transformer T into a primary side circuit 10 and a secondary side circuit 20. A controller 108 in the primary side circuit 10 can control the switch action of a power switch Q10 such that the input electricity may discontinuously pass through the transformer T for providing the secondary side circuit the load.
The controller 108 is used to control the switch action of the power converter. However, the controller 108 needs a supplying voltage to support a normal action of an internal control circuit thereof. As shown in FIG. 1, once the input voltage VIN is applied to the power converter, a voltage will be induced on an auxiliary winding Na of the transformer T, and the induced voltage on the auxiliary winding Na will charge a maintaining capacitor C28 for provision to the controller 108.
In the market, for conforming to the request of power saving, the controller 108 of the power converter mostly uses a variable frequency to achieve the green mode. In other words, when the load of the secondary side circuit is under different conditions, the power converter may be automatically adjusted to have a best frequency according to the variation of the load for achieving a best power saving efficiency. Therefore, when outputting a full load, the power converter may need a high efficiency, and when outputting no load, it may want to lower the loss. However, according to the present technology, when the output load of power converter is very low, the controller 108 always controls the switch behavior of the power switch Q10 in a burst mode such that the power converter can automatically reduce a switching frequency of the power switch Q10 and therefore reduce the loss at low load condition. The switching frequency in no-load is determined by the designer; in some examples, it may even be less than 500 Hz for conforming to the manufacturers' demands.
For current technology, the reason why power saving has a threshold is that the auxiliary winding Na of the transformer T provides the working voltage to the controller 108. Therefore, if some unnecessary dummy loads in the secondary circuit of the transformer T are reduced, for example, reducing the illumination of LED power indicator and adjusting an IC peripheral resistor parameter used in the secondary side circuit of the transformer T, then it may only need the necessary current for maintaining a normal function of IC so as to reduce unnecessary losses. However, this method may induce a voltage lower than the normal working voltage of the controller 108 on the auxiliary winding Na of the transformer T so as to cause a vibration of the output voltage. The reason is since all the energy is transmitted by one identical iron core of the transformer T, when the secondary side current of the transformer T becomes smaller, the induced current of the auxiliary winding Na may also induce a smaller current so as to cause the current charging the maintaining capacitor C28 to become smaller (namely, Vc28 is reduced). Therefore, using the conventional power supplying circuit to supply the controller 108 for power saving will necessarily suffer some congenital limits.